In hydrocarbon exploration surveys, such as gravity surveys, a great deal of time and money is spent acquiring data. In many areas, surveys have already been performed and use of existing data is desirable from an efficiency viewpoint. However, data acquired from previous surveys may have a biasing factor throughout.
This problem generally pertains to data sets recorded several decades ago or recorded by different companies in more recent years. The reasons for the incompatibilities may be many. They could be due to: a difference in datums; an inconsistency in parameters utilized for standard data reduction procedures (such as density in the Bouguer and topographic corrections for gravity data); a difference in the quality of the recording instruments; a difference in the skills of the recording personnel; a difference in assigning values to those parameters more dependent on subjective estimations (such as the average elevations in a zone of the Hammer chart used to determine topographic corrections for gravity data); etc. As can be seen, some of the above reasons for the data mismatch may result in systematic errors or unequal bias while others may result in random errors or variable bias.
One method to alleviate data set mismatch at a common boundary is to adjust the data of one area by a constant to bring those data to the level of the second area. This type of adjustment is satisfactory in many, but not all, cases. The constant of adjustment is determined from an average of the difference of the values of the two data sets in a narrow area along their boundary. This narrow area is referred to as the artificial overlap zone or just the overlap zone.
If data adjustment by a constant shift is unacceptable, the previously recorded data for an adjacent area can not be used and new data has to be measured.